When you attempt to access the ArcGIS Server Administrator Directory on a site that is federated with a portal, you must either provide the primary site administrator name and password or use OAuth credentials to login through your portal.
See OAuth credentials (for user authentication) for information about how to implement authentication using OAuth credentials in your portal.
When you attempt to access the ArcGIS Server Services Directory on a site that is federated with a portal, you can log in using your portal credentials. You cannot log in to the Services Directory using the primary site administrator name and password.